In conventional switching power supplies, an inductive element is usually provided which is utilized to store energy on a periodic basis by connecting it to a power source through a switching element, which stored energy is then transferred to a load. The switching element is controlled through a control device that detects the output voltage or current and then changes the on/off duty cycle of the switching element to vary the output voltage or current level, depending upon whether the regulation is current or voltage.
Most switching power supplies utilize a fairly high frequency for the switching operation to reduce the size of the magnetics. This provides for a lighter and lower cost power supply. However, the considerations that are prominent in the power supply designer's mind are directed toward the efficiency of the design, the heat generated by the design and the peak currents in the design.
In some designs, a transformer is utilized to provide the initial transformation from a high input AC voltage to an intermediate AC voltage and then the switching performed on the output of the transformer. In this type of design, the transformer is typically optimized to reduce the leakage inductance in the transformer by tightly coupling the primary and secondary windings in the transformer to reduce the leakage inductance, and minimizing the series resistance, to reduce heat dissipation in the transformer. The primary reason is to reduce the size as much as possible and achieve a much more efficient low temperature transformer for a given application. For switching transformer applications, this typically will require an additional switching inductor for use with a higher frequency application, since the primary transformer typically operates at the line frequency of the input signal.
The primary disadvantage with most systems that utilize a primary transformer for reducing the voltage on the input to an intermediate AC voltage and then utilizing a conventional switching power supply circuit for regulation is the additional inductive element required for the switching operation. This results in an increased number of parts and complexity of the design.